DENVER, Sept 16 (Reuters) - Seven people were confirmed dead and at least 1,500 homes destroyed in Colorado after a week of rare, torrential rains along the eastern slopes of the Rockies, and helicopter search-and-rescue flights resumed on Monday in flood-stricken areas.

Much of the evacuation effort was focused on remote foothill and canyon communities of Larimer and Boulder counties in north-central Colorado, where 1,000 residents remained stranded due to washed-out roads, bridges and communication lines, the county sheriff's office said.

Drizzle and patchy morning fog that had hampered airborne emergency operations lifted by afternoon, allowing National Guard helicopters to return to the skies to help ground teams find trapped flood victims and carry them to safety.

Ranchers were advised to move livestock away from rain-swollen streams as floodwaters spread further east onto the prairie, and authorities warned residents to be on the lookout for rattlesnakes that might be moving to higher ground.

Larimer and Boulder counties bore the brunt of flash floods first unleashed last week by heavy rains that started last Monday and drenched Colorado's biggest urban centers along a 130-mile (210 km) stretch in the Front Range of the Rockies.

At the peak of the disaster, the heaviest deluge to hit the region in four decades, floodwaters streamed down rain-saturated mountainsides northwest of Denver and spilled through canyons funneling the runoff into populated areas below.

The flooding progressed downstream and spread onto the prairie on Friday. During the weekend, waters topped the banks of the South Platte River and inundated farmland as high water rolled eastward in the direction of Nebraska.

SAND-BAGGING RAILROAD TRACKS

State officials issued flood warnings to Nebraska residents along the South Platte. State emergency management spokeswoman Jodie Fawl said they began putting sandbags inside culverts beside a Union Pacific Railroad line in the town of Big Springs to prevent a wash-out of the tracks there.

The Colorado Office of Emergency Management issued a statement on the disaster, putting the official death toll at seven, up from five over the weekend, but a breakdown of the fatalities was not immediately available.

Separately, two women, aged 60 and 80, remained missing and presumed dead after their homes were washed away by flash flooding in the Big Thompson Canyon area, Larimer County sheriff's spokeswoman Jennifer Hillmann said. But she said local authorities were still not counting those two women as confirmed dead because their bodies had not been recovered.

Nearly 400 other people remain unaccounted for in Larimer County, with many believed to be still stranded in remote areas cut off by floodwaters and left without telephone, cell phone or Internet service, she said.

An estimated 1,500 homes have been destroyed and 4,500 more damaged in Larimer County alone, Hillmann said. In addition, 200 businesses have been lost and 500 damaged, she said, citing preliminary assessments by the county.

As the weather began to clear Sunday night and Monday, rescue workers fanned out across a flood zone encompassing an area nearly the size of Delaware.

"They'll take advantage of the weather today and help out everyone they can," said Micki Trost, a spokeswoman for the state Office of Emergency Management. "We hope that those weather forecasts stay in our favor."

The air rescue operations were the largest in the United States since flooding in the aftermath of Hurricane Katrina devastated New Orleans in 2005, National Guard officials said.

Bryon Louis of the National Weather Service office in Boulder said some areas had been soaked by as much as 16 inches (40 cm) of rain in just three days, the average for an entire year in the semi-arid region.

President Barack Obama declared the area a major disaster over the weekend, freeing up federal funds and resources to aid state and local governments.

U.S. Army and National Guard troops have rescued 1,750 people cut off by washed-out roads in the mountain canyons of Boulder and Larimer counties, Army spokesman Major Earl Brown said in a statement.

State officials would be unable to assess the overall damage until rescue efforts were complete and the floodwaters had receded, Trost said.

The prolonged showers were caused by an atmospheric low-pressure system that stalled over Nevada and western Utah, drawing extremely moist air out of Mexico and streaming it north into the southern Rockies, meteorologists said.

The last multi-day rainfall event to spawn widespread flooding in Colorado's Front Range occurred in 1969. But a single-night deluge from a 1976 thunderstorm triggered a flash flood that killed more than 140 people in Big Thompson Canyon. (Reporting by Keith Coffman; Writing by Steve Gorman; Additional reporting by Tom Brown; Editing by Dan Whitcomb, Richard Chang and Ken Wills)

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